Numerous products in the high tech area include generally planar components that must be subjected to a number of sequential processing steps. Examples of objects processed in this manner include flat panel displays, optical and magnetic recording disks, photomasks, and silicon wafers from which semiconductor chips are fabricated. For the sake of simplicity, the present invention is discussed below with reference to the manufacture of semiconductor devices from silicon wafers. However, this is not intended to limit in any way the scope of applications to which the cassette of the present invention may be applied.
Surface processing of silicon wafers to make semiconductor devices such as integrated circuits involves a number of stages, with the wafers being exposed to many different processing steps. Wafers are placed in process vessels where they are repeatedly subjected to heated chemicals, such as sulfuric acid. The wafers are also rinsed and cleaned between processing steps. For optimum process performance, it has become customary for the wafers to be confined within specially-constructed relatively low mass cassettes having customized interfaces with the process vessels and/or with the robots or other machinery used to transport the cassettes during processing.
In general, the wafers are arranged in a linear array configuration within the cassette, with sufficient space between each wafer to allow exposure to the various chemicals necessary for processing, and to prevent contact between adjacent wafers. The cassette is typically rigid in order to maintain the required spacing between the wafers, even under the relatively high temperatures to which the cassette is exposed.
One of the most common wafer cassette designs currently used is the rigid four-bar cassette shown in FIGS. 1A-1D. Such a prior art cassette is described in U.S. Pat. No. 4,872,554. FIGS. 1A-1C show perspective, top, and end views, respectively, of a conventional rigid four-bar wafer cassette. Rigid endplates 2 secure the ends of each of the rigid bars or rods 3. Rigid bars 3 have combs or "teeth" 4 adapted to accept and separate inserted wafer(s) 5. During use of the four-bar cassette, an inserted wafer is intended to come into contact with the cassette 1 at four different points 6, as shown in FIG. 1D. The cassette and wafers are lowered into a process vessel V for treatment of the wafers.
While considered a great improvement over more conventional cassettes, the four-bar cassette leaves further room for improvement. For example, in the semiconductor fabrication industry it is desirable to minimize the volume of process vessels, as this in turn minimizes the volume of processing chemicals and rinse fluids needed to fill the vessels. It is for this reason that the wet processing vessels are often designed to conform as closely as possible to the size and shape of the wafers undergoing processing. Development of the four-bar cassette was a step towards reducing the outer profile of the cassette. However, the support bars and end plates of the four-bar create a relatively large outer profile for the cassette, which still must be accommodated by a fairly large vessel interior. Handles 7, which are engaged by a wafer handling system during transport of the cassette, further contribute to the bulk of the cassette.
It is thus desirable to configure a wafer cassette having a further reduced outer profile so as to allow for the use of even smaller processing vessels and thus smaller quantities of process fluids.
Another area of improvement relates to the "shadowing" effects that cassettes can have on rinse fluids and/or megasonic energy that is directed towards the wafers during processing. During cleaning applications in which the wafers are immersed in a cleaning solution, megasonic energy may be directed towards the wafers using one or more megasonic transducers. The resulting agitation of the cleaning solution is sufficiently powerful to remove particles from the surfaces of the wafers.
Typically, the megasonic transducers are oriented to direct megasonic energy towards the wafers from the bottom of the process vessel V. Because the four-bar cassette contacts each wafer at four points along the lower wafer edge (see FIG. 1D), the bars obstruct megasonic energy and can impair the effectiveness of the megasonic cleaning process. Many rinse tanks come equipped with specialized megasonic transducer arrangements designed to minimize the effects of the cassette structure on the megasonic energy patterns.
A similar shadowing phenomenon can occur during cleaning or rinse procedures in which fluids are directed upwardly into the vessel from the vessel bottom. Such shadowing effects can prevent the fluids from adequately reaching certain regions of the wafer surface, leaving undesirable streaks of particles. It is thus further desirable to provide an improved cassette designed to minimize obstruction of megasonic energy and rinse fluid flow patterns by cassette components.
A better understanding of the features and advantages of the present invention will be obtained by reference to the detailed description and accompanying drawings given below. The following description and drawings illustrate a rigid cassette useful for transporting silicon wafers during semiconductor processing steps. However, it is important to recognize that the cassette design shown and described herein represents only one particular embodiment utilizing principles of the present invention.